OLEDs have been known for approximately two decades. All OLEDs work on the same general principles. One or more layers of semiconducting organic material is sandwiched between two electrodes. An electric current is applied to the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charge, typically referred to as holes, moves from the anode. The positive and negative charges meet in the center layers (i.e., the semiconducting organic material), combine, and produce photons. The wave-length—and consequently the color—of the photons depends on the electronic properties of the organic material in which the photons are generated.
Light emitting devices, which may be generally classified as organic or inorganic, are well known in the graphic display and imaging art. Among the benefits of organic light emitting devices are high visibility due to self-emission, as well as superior impact resistance, and ease of handling of the solid state devices. Organic light emitting devices may have practical application for television and graphic displays, as well as in digital printing applications.
An OLED is a thin film structure formed on a substrate. A light emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The light emitting layer may consist of multiple sublayers. When a potential difference is applied across the device, negatively charged electrons move from the cathode to the electron-injecting layer and finally into the layer(s) of organic material. At the same time, positive charges, typically referred to as holes, move from the anode to the hole-injection layer and finally into the same light emitting organic layer. When the positive and negative charges meet in the organic material layer(s), they recombine and produce photons. The wave length—and consequently the color—of the photons depends on the electronic properties of the organic material in which he photons are generated.
In a typical matrix-addressed OLED device, numerous OLEDs are formed on a single substrate and arranged in groups in a grid pattern. Several OLED groups forming a column of the grid may share a common cathode, or cathode line. Several OLED groups forming a row of the grid may share a common anode, or anode line. The individual OLEDs in a given group emit light when their cathode line and anode line are activated at the same time.
Fabrication of color displays generally requires side-by-side patterning of red, green and blue (“RGB”) sub-pixels. Since the OLED devices are extremely moisture sensitive, any type of wet processing is normally not possible. Moreover, suitable red, blue and green color emitter materials, with good color gamut and lifetime, have not yet been realized. As such, most color OLED displays are fabricated using either color filters or color changing media (“CCM”).
A problem arises in the case of CCM materials, wherein, the emitted fluorescent light can be trapped inside the CCM film as well as the transparent substrate on which the CCM film is deposited. This problem is due to the effect of total internal reflection (wave-guiding effect), which is significant if the angle of incidence exceeds the critical angle for that material. The effect is observed when the light exits out of a medium with a higher index of refraction and enters a medium with a lower index of refraction. This situation is identical to the case of a CCM coating over a glass substrate. As a result of light trapping the total amount of useful light (fluorescent) obtained from the CCM material is vastly reduced. FIG. 1 shows an example of light trapping effect.